On some receipts, securities, and certificates, a special pattern which emerges as a character string or image when they are copied is printed on the background, so as to prevent them from being readily copied. Such special pattern is generally called a “copy-forgery-inhibited pattern”, which applies a mechanism that does not allow an original to be readily copied, thus psychologically inhibiting a copy of an original.
This copy-forgery-inhibited pattern is formed of two regions, i.e., a region where dots remain after copying and a region where dots disappear after copying. These two regions have substantially the same densities, and a hidden character string or image such as “COPY” cannot be seemingly recognized macroscopically, but these regions microscopically have different properties. Note that the hidden character string or image will be referred to as a “latent-image” hereinafter.
For example, the region where dots remain after copying (to be referred to as a latent-image part hereinafter) is formed of groups of dots where dots are concentrated, and the region where dots disappear or become lighter than the density of the latent-image part of a copy after copying (to be referred to as a background-image part hereinafter) is formed of scattered dots. In this manner, the two regions which have substantially the same densities but different properties can be created.
The concentrated dots and scattered dots can be generated by halftoning using halftone dot screens with different screen ruling values or dithering using dither matrices having different features as an image process.
In halftoning, a halftone dot screen with a low screen ruling value is preferably used to obtain the concentrated dot layout, and a halftone dot screen with a high screen ruling value is preferably used to obtain the scattered dot layout.
In the dithering using a dither matrix, a dot-concentration dither matrix is preferably used to obtain the concentrated dot layout, and a dot-scattering dither matrix is preferably used to obtain the scattered dot layout.
Therefore, when a copy-forgery-inhibited pattern image is generated using the aforementioned halftoning, halftoning with a low screen ruling value is suited to a latent-image part, and halftoning with a high screen ruling value is suited to a background-image part. When a copy-forgery-inhibited pattern image is generated using the aforementioned dithering, dithering using a dot-concentration dither matrix is suited to a latent-image part, and dithering using a dot-scattering dither matrix is suited to a background-image part.
In general, a copying machine suffers a limitation on image reproduction performance depending on the input resolution upon reading small dots of a document to be copied or the output resolution upon reproducing small dots. Therefore, when a document includes isolated small dots beyond the limitation on the image reproduction performance of the copying machine, these small dots cannot be perfectly reproduced on its copy, and the isolated small dots disappear.
That is, when the background-image part of the copy-forgery-inhibited pattern is designed to exceed the limitation of dots that can be reproduced by the copying machine, large dots (concentrated dots) of the copy-forgery-inhibited pattern can be reproduced by copying, but small dots (scattered dots) cannot be reproduced. Hence, a hidden image (latent-image) emerges. Also, even when scattered dots do not completely disappear after copying, if they have an apparent density difference from concentrated dots after copying, a hidden image (latent-image) emerges.
In the copy-forgery-inhibited pattern, a technique called “camouflage” which makes it harder to recognize a hidden character string or image as a latent-image is well known. This camouflage technique is a method of laying out a pattern which has a density different from that of the latent-image part and background-image part on the entire copy-forgery-inhibited pattern image. This technique has an effect of macroscopically emphasizing the camouflage pattern with a density different from that of the latent-image part and background-image part, and further obscuring a latent-image at a glance.
The copy-forgery-inhibited pattern with the camouflage pattern has an effect of giving a decorative impression to a print compared to a copy-forgery-inhibited pattern without any camouflage pattern. Dots inside the camouflage pattern preferably disappear as much as possible so as to allow easy recognition of a latent-image after copying. In case of the simplest implementation, camouflage can be realized by printing no dots at positions corresponding to the camouflage pattern.
An overview of the copy-forgery-inhibited pattern has been explained.
Conventionally, a print paper vendor prints a copy-forgery-inhibited pattern including a character string or image (latent-image) such as “copy” or the like on dedicated sheets, and sells such sheets as copy-inhibition paper sheets. The government and other public offices, and companies buy such copy-inhibition paper sheets, and print documents whose authenticity is to be guaranteed on copy-inhibition paper sheets, thus inhibiting copies of prints.
Since the aforementioned copy-inhibition paper sheets are prepared by pre-printing a copy-forgery-inhibited pattern on dedicated sheets by a print paper vendor, they have demerits in terms of cost such as cost produced upon using dedicated sheets, cost produced upon preparing pre-print sheets more than necessary, and the like.
By contrast, in recent years, a technique for creating a copy-forgery-inhibited pattern image by software, and outputting a document with that copy-forgery-inhibited pattern image on its background using a laser printer (to be referred to as “on-demand copy-forgery-inhibited pattern output method by printer” hereinafter) has been realized (e.g., see patent reference 1: Japanese Patent Laid-Open No. 2001-197297).
With this on-demand copy-forgery-inhibited pattern output method by a printer, since a document with a copy-forgery-inhibited pattern image on its background can be printed using plain paper, only a required number of documents with a copy-forgery-inhibited pattern image on their backgrounds can be printed when needed. Therefore, copy-inhibition paper sheets need not be prepared more than necessary unlike in the conventional method. That is, the on-demand copy-forgery-inhibited pattern output method by a printer can greatly reduce cost of sheets compared to the conventional document copy-inhibition method using copy-inhibition paper sheets.
The user of the conventional copy-inhibition paper sheets can use only a hidden character string or image (latent-image) prepared in advance or a made-to-order hidden character string or image (latent-image).
However, with the on-demand copy-forgery-inhibited pattern output method by a printer, the user can generate a copy-forgery-inhibited pattern image including an arbitrary hidden character string or image (latent-image) by a software process for each print, and can print it on-demand using a printer. Hence, the user can freely customize a hidden character string or image (latent-image).
By exploiting a merit of on-demand selection of a latent-image, not only a corporation logo mark or a character string of copy inhibition, which is used conventionally, but also various kinds of information such as a serial number or IP address used to identify an output printer, a computer name or IP address used to identify a computer that issues a print command, a user name or login name used to identify a user who issues a print command, a print job number, print date, print location, the file name of a digital document, and the like used to identify when and by whom a print process is done, and so forth can be selected as an image or character string to be embedded as a latent-image.
As a result, the on-demand copy-forgery-inhibited pattern output method by a printer can implement an advanced tracking function that cannot be implemented by the conventional pre-printed copy-inhibition paper sheets.
In the on-demand copy-forgery-inhibited pattern output method by a printer, variations of the densities of the latent-image part and background-image part depending on the printer engine characteristics, print environment (temperature and humidity), and the state of output paper sheets (media), and the like are predicted.
In order to lay out a copy-forgery-inhibited pattern image on the background of a document whose copies are to be inhibited, and to implement a copy-inhibition effect, the densities of the latent-image and background-image parts of the copy-forgery-inhibited pattern image must be appropriately adjusted, so that the latent-image and background-image parts of the copy-forgery-inhibited pattern image have nearly equal densities, and the latent-image part in a copy emerges after copying. In this specification, parameters used to determine the print densities of the background and latent-image parts of a copy-forgery-inhibited pattern image upon generating the copy-forgery-inhibited pattern image will be referred to as “copy-forgery-inhibited pattern density parameters”. Note that elements of the copy-forgery-inhibited pattern density parameters will be described in detail later.
As a method of finding out the copy-forgery-inhibited pattern density parameters, a test printing process is executed before a document and copy-forgery-inhibited pattern image are composited. The user checks this test print to visually find out a copy-forgery-inhibited pattern image in which the latent-image and background-image parts have substantially equal densities upon printing, and a latent-image emerges after copying by a target copying machine. Then, parameters used upon generating the found copy-forgery-inhibited pattern image are determined as optimal copy-forgery-inhibited pattern density parameters. Such method is normally used. After that, a copy-forgery-inhibited pattern image is generated based on the determined optimal copy-forgery-inhibited pattern density parameters, and is composited to the document who copies are to be inhibited, thus printing out a composite image.
However, some printers have largely different halftone reproduction characteristics of the latent-image and background-image parts due to the printer engine characteristics and individual differences. Also, the latent-image and background-image parts have different halftone reproduction characteristics owing to different types of output paper sheets. For example, in case of an ink-jet printer, the latent-image and background-image parts may have different halftone reproduction characteristics due to blurring of dots depending on the characteristics of output paper sheets. Some printers have large density variations depending on a print environment and aging. For example, in case of a laser printer, the electric field distribution of a photosensitive drum is susceptible to changes in the humidity and temperature. When the electric field distribution becomes broad and smooth, reproduction of scattered dots or isolated dots becomes unstable.
In general, in the test printing process of a copy-forgery-inhibited pattern image, one or both of the densities of the latent-image and background-image parts must be changed little by little so that the density difference between the finally obtained latent-image and background-image parts is visually imperceptible, thus adjusting one or both densities.
When the halftone reproduction characteristics of the latent-image and background-image parts that depend on the printer engine characteristics, output paper sheets, and the like are determined, optimal copy-forgery-inhibited pattern density parameters can be determined in correspondence with a model or paper sheet involved.
However, upon finding out optimal copy-forgery-inhibited pattern density parameters in consideration of changes in densities of the latent-image and background-image parts depending on the print environment, aging, and the like, if a density change width produced as a result of a change in print environment or aging is also considered, a large number of combinations of copy-forgery-inhibited pattern density parameters are prepared by dividing a relative broad range from lower to higher densities in increments of a change width as slight as the density difference between the latent-image and background-image parts cannot be visually recognized, and undergo a test printing process, so as to find out optimal copy-forgery-inhibited pattern density parameters.
In case of a printer which suffers a large density variation width depending on a change in print environment or aging, the number of combinations of copy-forgery-inhibited pattern density parameters required to find out an optimal copy-forgery-inhibited pattern image increases inevitably according to the density variation width. It is possible to execute test printing processes of all copy-forgery-inhibited pattern density parameters, but many paper sheets are wasted by the test printing processes.
Therefore, in a printer which has unknown halftone reproduction characteristics of the latent-image and background-image parts depending on its engine characteristics or output paper sheets (e.g., a printer of an old generation or a printer in the future), or a printer which suffers a large density change of the latent-image and background-image parts depending on the print environment, aging, and the like, it is difficult to obtain an optimal copy-forgery-inhibited pattern image by a single test printing process. For this reason, in the on-demand copy-forgery-inhibited pattern output method by a printer, printers that can output a copy-forgery-inhibited pattern image are limited.
However, if there is means for efficiently finding out optimal copy-forgery-inhibited pattern density parameters, and that means is provided to users who print a copy-forgery-inhibited pattern using a copy-forgery-inhibited pattern generated by the on-demand copy-forgery-inhibited pattern output method by a printer, there is a merit of generally applying the on-demand copy-forgery-inhibited pattern output method by a printer to more printers.
On a conventional copy-inhibition paper sheet, the densities of the latent-image and background-image parts are fixed, and the density of a copy-forgery-inhibited pattern image cannot be changed in correspondence with document data. However, when the latent-image and background-image parts of a copy-forgery-inhibited pattern have nearly equal densities upon printing, and the copy-forgery-inhibited pattern has a nature that a latent-image emerges when that copy-forgery-inhibited pattern is copied by a target copying machine, the density can be freely selected.
Since the aforementioned on-demand copy-forgery-inhibited pattern output method by a printer can easily change the density of a copy-forgery-inhibited pattern, a creator of the copy-forgery-inhibited pattern can freely select the density of the copy-forgery-inhibited pattern as a merit.
For example, the density of the copy-forgery-inhibited pattern can be changed by increasing or decreasing the gray level of a multi-valued background before execution of halftoning or error diffusion using a scheme described in the on-demand copy-forgery-inhibited pattern output method by a printer described in patent reference 1. An application that composites and outputs a copy-forgery-inhibited pattern has a user interface that prompts the user to select the density of a copy-forgery-inhibited pattern, and makes the user select the copy-forgery-inhibited pattern density. The application executes a test printing process at the selected density, and can determine parameters which can make the densities of the latent-image and background-image parts nearly equal to each other upon printing.
However, in case of the above implementation, at the density of a copy-forgery-inhibited pattern selected by the creator of the copy-forgery-inhibited pattern from the user interface, the background may not disappear upon copying due to too high a density, or parameters at which a latent-image emerges upon copying may not be found due to too low a density. In this case, the creator of the copy-forgery-inhibited pattern must select another density of the copy-forgery-inhibited pattern, execute a test printing process, and copy the printed copy-forgery-inhibited pattern. Therefore, the sequence for finding out optimal copy-forgery-inhibited pattern parameters is troublesome for the creator of the copy-forgery-inhibited pattern.